The present invention relates to a position detecting apparatus and method, and, more particularly, to a position detecting apparatus and method for detecting a position of an object, such as a wafer, in an exposure apparatus that manufactures various devices, including semiconductor chips, such as ICs and LSIs, CCDs, and magnetic heads. The present invention is suitable, for example, for performing alignment between a reticle and a wafer.
Recent demands for smaller and thinner electronic apparatuses have increasingly required finer processing to semiconductor devices mounted on these electronic apparatuses. A reduction projection exposure apparatus has been conventionally employed, which uses a projection optical system to project a circuit pattern formed on a mask (or a reticle) onto a wafer, etc., to transfer the circuit pattern, in photolithography technology for manufacturing the semiconductor device.
The minimum critical dimension (“CD”) to be transferred by the projection exposure apparatus or resolution is proportionate to a wavelength of light used for exposure, and inversely proportionate to the numerical aperture (“NA”) of the projection optical system. The shorter the wavelength is, the better is the resolution. Therefore, recent demands for finer processing to semiconductor devices have promoted the use of a shorter wavelength of the UV light from an ultra-high pressure mercury lamp (i-line with a wavelength of about 365 nm) to a KrF excimer laser (with a wavelength of about 248 nm) and an ArF excimer laser (with a wavelength of about 193 nm). More recently, the light having a shorter wavelength is reduced in practice, such as an F2 laser (having a wavelength about 157 nm) as ultraviolet (“UV”) light having a shorter wavelength and extreme (“EUV”) light.
In order to satisfy these requirements, a step-and-repeat exposure apparatus (also referred to as a “stepper”) for projecting and exposing an approximately square exposure area all together onto a wafer with a reduced exposure area has been replaced mainly with a step-and-scan exposure apparatus (also referred to as a “scanner”) for accurately exposing a wide screen of exposure area through a rectangular or arc slit while relatively and quickly scanning the reticle and the wafer.
In exposure, the scanner uses a surface position detector in an oblique light projection system to measure a height (or a surface position) of a certain position on the wafer before the exposure slit area reaches the certain position on the wafer, and correctly positions the wafer surface at the best focus position when exposing the certain position.
In particular, the exposure slit area has plural measurement points in the longitudinal direction of the exposure slit, i.e., a direction perpendicular to the scan direction, to measure an inclination (tilt) of the surface as well as a height (focus) of the surface position of the wafer. Japanese Patent Application, Publication No. 6-260391 (corresponding to U.S. Pat. No. 5,448,332), discloses, for example, a method of measuring the focus and tilt.
The projection exposure apparatus requires a highly precise alignment that performs alignment between the reticle and the wafer, along with the fine processing to the circuit pattern (or the improvement of the resolution). The necessary precision for the alignment is generally about one-third of a circuit critical dimension and, for example, which is 60 nm on one-third as along as the current design width of 180 nm.
In the alignment between the reticle and the wafer in each shot, positions of alignment marks that are exposed on the wafer for each shot are detected simultaneously with a detection of a circuit pattern on the reticle. The position of the alignment mark is detected by receiving the light from the alignment mark via an optical system and a CCD camera, signal-processing the obtained electrical signal using various parameters, and positioning the wafer relative to the reticle based on the detection results. See, for example, Japanese Patent Application, Publication No. 6-260390 (corresponding to U.S. Pat. No. 5,703,685). The detection results include, for example, a magnification of a wafer, a rotation of a wafer, a shift amount, etc.
The recent, increasingly shortened wavelength of the exposure light and the higher NA of the projection optical system have required an extremely small depth of focus (“DOF”) and stricter accuracy with which the wafer surface to be exposed is aligned to the best focus position. This accuracy is also referred to as the focus accuracy. A measurement error of a surface position detecting means cannot become negligible. The measurement error results from influence of a pattern on a wafer and an uneven coating thickness of the resist applied to the wafer.
For example, the uneven coating thickness of the resist generates a step near a peripheral circuit pattern and a scribe line. The influence of the uneven coating thickness of the resist is smaller than the DOF, but is significant to the focus measurement. This step enlarges an inclined angle on a resist surface, and causes the reflected light detected by the surface position detecting means to offset from a regular reflection angle due to the reflections and refractions. The pattern density on the wafer causes a reflectance difference between a pattern crowded area and a coarse area. Since the reflected light detected by the surface position detecting means changes the reflecting angle and intensity, the detected waveform obtained from this reflected light contains an asymmetry that causes a measurement error, and a surface position on the wafer cannot be detected due to the remarkably lowered contrast in the detected waveform.
In general, the wafer process causes non-uniform pattern steps on a wafer surface and uneven coating thickness of the resist, resulting in poor reproducibility in one wafer and among wafers, and making the offset process difficult. This causes the exposure to stop, since the surface position of the wafer cannot be measured during the exposure, and a large defocus results in defective chips or lowered yield.
It is vital for the present semiconductor industry to improve the overlay accuracy of the wafer for high semiconductor device performance and manufacture yield (or throughput). However, the wafer induced shift (“WIS”) causes a large error (an asymmetry of an alignment signal) in a detection result, and deteriorates the alignment accuracy. The factors include an asymmetry of an alignment mark and an asymmetry of the resist.
Various methods are proposed as measures for the asymmetry of the alignment mark. For example, one method (1) maintains a wide illumination wavelength width for reduced influence of the thin film interference on the alignment mark. Another method (2) evaluates an exposure result after the alignment using prior several (send-a-head) wafers, and exposes subsequent wafers by feeding back the alignment offset amount to the exposure apparatus. Still another alignment method (3) uses a wavelength that irradiates plural lights having different wavelengths, and selects, for the alignment, one of the lights whose wavelength provides the maximum contrast in the alignment signal.
However, in order for the method (1) to completely eliminate the influence of the thin film interference of the resist, the method (1) requires a wavelength width twice as wide as the current illumination wavelength width of about 200 nm for the future thinning resist thickness, such as 200 nm to 300 nm. While the alignment optical system should correct the chromatic aberration for the wavelength width, it is technically very difficult to simultaneously correct the imaging positions and imaging magnifications for respective wavelengths. Therefore, the method (1) is unlikely to be viable.
The method (2) needs non-product wafers (“NPW”) for offset calculations, and deteriorates the cost performance of the device manufacture, because a 300 mm wafer that has already been reduced to practice is so expensive per sheet. Moreover, the fluctuation of the manufacture process varies steps and width of the alignment mark, and causes scattering errors that cannot be eliminated by the alignment offset.
The method (3) is likely to be implemented without increasing the NPW. However, the wavelength that provides the highest contrast in the alignment signal is not always closest to the wavelength that provides the least asymmetric errors of the alignment mark. Therefore, the alignment that uses the light having the wavelength that maximizes the contrast in the alignment signal dos not always minimize the errors caused by the asymmetry of the alignment mark.
There are demands for a position detecting method and apparatus and an exposure apparatus, which reduce the influence caused by the asymmetry of the alignment mark and provide highly precise position detections. In addition, there are other demands for an exposure method and apparatus, which realize a high focus accuracy relative to a small DOF, and improve the yield.